Switched capacitor circuits and operational amplifiers are widely used in both fixed and adjustable filters. Such switched capacitor type filters generally rely upon the effective resistance of a switched capacitor circuit to set filter coefficients. As is well known in the art, the effective resistance of a switched capacitor circuit is determined by the value of the capacitor and the frequency of the clock signals controlling the associated switches.
In filter applications of switched capacitor circuits, the frequency of the circuit clock signals is generally constrained to be substantially higher than the frequency of the signals processed by the filter. When switched capacitor filters employ more than one clock frequency, the higher frequency clock rate must be a integral multiple of the lower clock rate. Accordingly, varying the filter clocking frequency is generally not a practical means of adjusting filter parameters.
Some prior art circuits have employed banks of capacitors and switches to vary the capacitance of a switched capacitor in steps. Such arrangements require a very large number of steps to keep the quantizing noise induced by switching within acceptable limits. Furthermore, the digital control circuits for effecting the required switching are complex, and large numbers of capacitors in an integrated circuit switched capacitor filter make very inefficient use of chip area. The variation of capacitance by stepped increases or through changes in clock frequency allow only discrete step variations. It is preferred in many filter applications that the capacitance and the resulting effective resistance variations be continuously adjustable.